For instance, a gas sensor, which outputs an electromotive force, is provided at a vehicle engine (e.g., an automobile engine). In this type of gas sensor, exhaust gas, which is discharged from the engine, serves as a sensing subject of the gas sensor, and an oxygen concentration of the exhaust gas is sensed with the gas sensor. This type of gas sensor includes an electromotive force (EMF) cell, which outputs an electromotive force signal that varies depending on whether the exhaust gas is rich or lean. Specifically, when an air-to-fuel ratio is rich, the electromotive force cell outputs the electromotive force signal of about 0.9 V. In contrast, when the air-to-fuel ratio is lean, the electromotive force cell outputs the electromotive force signal of about 0 V.
In this type of gas sensor, when the air-to-fuel ratio of the exhaust gas changes between rich and lean, a change in the sensor output may be disadvantageously delayed relative to an actual change in the air-to-fuel ratio. In order to improve the output characteristic of such a gas sensor, various techniques have been proposed.
For instance, JP2012-063345A (corresponding to US2012/0043205A1) discloses a gas sensor control apparatus, in which a constant current circuit is connected to at least one of a pair of sensor electrodes (i.e., two sensor electrodes). In this gas sensor control apparatus, when it is determined that a demand for changing the output characteristic of the gas sensor is present, a flow direction of the constant electric current is determined based on the demand. Then, the constant current circuit is controlled to induce a flow of the constant electric current in the determined direction. Through the supply of the constant electric current, the output characteristic of the gas sensor is appropriately controlled.
The gas sensor (the O2 sensor), which outputs the electromotive force, may be placed on a downstream side of a three-way catalyst in an exhaust conduit of the engine to sense rich/lean of the air-to-fuel ratio of the exhaust gas after purification of the exhaust gas with the three-way catalyst. In such a case, CO, HC (both being rich components) and NOx (lean component) of the exhaust gas can be purified with the three-way catalyst. However, a catalytic conversion efficiency of the three-way catalyst for converting, for example, NOx is rapidly deteriorated when the air-to-fuel ratio exceeds a predetermined air-to-fuel ratio around a theoretical air-to-fuel ratio (e.g., when the air-to-fuel ratio is located on a lean side of a catalytic conversion window of the three-way catalyst). According to the study conducted by the inventors of the present application, with respect to the catalytic conversion characteristic of the three-way catalyst, it is confirmed that an NOx outflow air-to-fuel ratio point, at which NOx begins to outflow from the three-way catalyst, is located on a rich side of an air-to-fuel ratio point, which forms an equilibrium point of the rich component and the oxygen at the three-way catalyst.
In view of the above points, even in the case where NOx outflows from the three-way catalyst on the downstream side of the three-way catalyst, the sensing of the lean based on the presence of the oxygen in the exhaust gas is not appropriately conducted at the gas sensor in some cases. Therefore, in the case where a control operation for limiting NOx emissions is carried out through use of the measurement signal of the gas sensor, undesirable NOx emissions may possibly unexpectedly occur.
In the prior art technique discussed above, the constant electric current is conducted between the two sensor electrodes to change the sensor output characteristic and thereby to improve the sensing response. Furthermore, at a high load operation of the engine, a response level is increased in view of an expected increase in the amount of NOx emissions in response to an increase in the amount of fresh air.
However, there is no prior art technique, which changes a sensor output characteristic in view of a catalytic conversion characteristic in the above described catalysts. It is considered that the technique, which is based on the new objective of changing the sensor output characteristic according to the catalytic conversion characteristic of the catalyst, has a significant meaning.